STEEL FREE FIBRE REINFORCED CONCRETE BRIDGE DECKS: NEW CONSTRUCTION AND REPLACEMENT

Transcription

1 APWA International Public Works Congress NRCC/CPWA Seminar Series Innovations in Urban Infrastructure STEEL FREE FIBRE REINFORCED CONCRETE BRIDGE DECKS: NEW CONSTRUCTION AND REPLACEMENT by Abstract Aftab Mufti Dalhousie University and Baidar Bakht JMB Structures Research Inc. Researchers at Dalhousie University, Halifax, Canada, and at the Ministry of Transportation of Ontario (MTO) have successfully enhanced the strength of deck slabs by further exploiting their arching action. With the help of tests on both large- and small-scale laboratory models, they have concluded that a deck slab supported by parallel longitudinal beams does not required any reinforcement, provided that the slab is suitably confined in both the longitudinal and transverse directions. In the longitudinal direction, the deck slab is confined by making it composite with the beams, and by edge beams with suitably high flexural rigidity in the plane of the slab. In the transverse direction, the confinement is provided by ensuring that the supports of the slab, i.e. the top flanges of the girders, are restrained effectively against relative lateral movement; such confinement can be provided, for example, by welding steel straps to the top flanges of the girders. The practice of patenting 'innovative concepts' in the field of structural engineering is not new. In the same tradition, the concepts of cast-in-place and precast steel-free deck slabs are protected through patents. The patent for cast-in-place steel-free deck slabs is already granted in USA, UK and France, and is pending in Canada, Germany and Japan. A global patent for precast arch panels without tensile reinforcement is pending. 1. Arching in Slabs A typical deck slab on girders at a spacing of about 2m, when designed for flexure, contains about 30 kg steel reinforcement per m 2 of the slab area. When it is designed by taking account of arching, steel reinforcement can be reduced to about 20 kg/m 2. The Ontario Highway Bridge Design Code (OHBDC), in its three editions (1979, 1983 and 1992) has incorporated an empirical design method for deck slabs, which takes account of the inherent arching action in deck slabs. Hundreds of deck slabs designed by this empirical method now exist not only in Ontario, but also in other parts of the world. Researchers at Dalhousie University, Halifax, Canada, and at the Ministry of Transportation of Ontario (MTO) have successfully enhanced the strength of deck slabs by further exploiting their arching action. With the help of tests on both large- and small-scale laboratory models, they have concluded that a deck slab supported by parallel longitudinal beams does not required any reinforcement, provided that the slab is suitably confined in both the longitudinal and transverse directions (Mufti et al., 1993; Bakht and Agarwal, 1995). In the longitudinal direction, the deck slab is confined by making it composite with the beams, and by edge beams with suitably high flexural rigidity in the plane of the slab. In the transverse direction, the confinement is provided by ensuring 63

2 that the supports of the slab, i.e. the top flanges of the girders, are restrained effectively against relative lateral movement; such confinement can be provided, for example, by welding steel straps to the top flanges of the girders. 2. Design Provisions Design provisions for steel-free deck slabs, included in the forthcoming Canadian Highway Bridge Design Code (CHBDC), required that a steel-free deck slab have a minimum thickness of the greater of 175 and one-fifteenth of the beam spacing. The traverse confining system of steel-free deck slabs include straps, each of which is required to have minimum cross-sectional area, A, in 2, given by: 2 F s S A = E t where the factor F s is 6.0 for outer panels and 5.0 for inner panels of the deck slab. It is noted that F s has the units of MPa; the spacing of the supporting beams, S, is in m; the spacing of the straps, S l, is in m; the modulus of elasticity of the strap material, E, is in MPa, and the thickness of the deck slab, t, is in. The presence of E in the denominator of the right-hand side of the above equation confirms that this requirement relates to the stiffness, rather than the strength, of the straps. To control the cracking of concrete which occurs in its early life, randomly-distributed lowmodulus fibres are mixed with concrete. The CBBDC provides a numerical measure for the minimum requirement for these fibres. It is noted that the addition of low-modulus fibres does not increase the tensile strength of concrete. The CBBDC Technical Subcoittee drafting the design provisions for steel-free deck slabs was concerned that the un-reinforced concrete deck slab might lack resistance against lateral stresses generated at the longitudinal sections of the slab on each side of the beams; these stresses, which can cause the failure illustrated in Fig. 1, are denoted herein as the 'separating stresses.' Fig. 1: Failure by separating stresses The issue of the separating stresses in the steel-free deck slab was resolved by the work of Razaqpur and Ghali (1984), which shows that these stresses cannot be calculated directly. This paper, based on extensive finite element analyses, provides a graphical method of determining the separating stresses in concrete T-beams. From this work, it was found that in a T-beam subjected to a uniformly distributed load of a given intensity, the separating stresses remain independent of the span 64

3 length irrespective of the fact that the horizontal shear force does increase with the beam span. By using the method of Razaqpur and Ghali, the separating stresses were calculated under realistic loads in T-beams incorporating bridge girders and a 175 nun thick composite flange of un-reinforced concrete. These stresses were found to be significantly smaller than the tensile strength of concrete. In a steel-free deck slab, the T-beams are not isolated. Even if the concrete cracks as a result of the separating stresses, the straps would be able to maintain the integrity of the structure by taking the resulting tension themselves. It was thus concluded that there was no need to require the calculation of separating stresses in steel-free deck slabs. The distress-free performance of the steel-free deck slabs of five bridges in Canada under unrestricted traffic has given solid support to the validity of this conclusion. These bridges are described in the following. 3. Five steel-free deck slabs in Canada 3.1 Salmon River Bridge The first steel-free deck slab was cast in October, 1995, on the steel plate girders of the Salmon River bridge in Nova Scotia, Canada. The bridge has two simply-supported spans, each 31.2 m long and with a skew angle of 22º. One of the spans comprises a 225 thick concrete slab, with steel reinforcement designed by the empirical method of the OHBDC. The other span has a 200 thick concrete slab which has no tensile reinforcement, but contains chopped polypropylene fibres randomly mixed at a volume fraction of 0.55 %. As shown in Fig. 2, this steel-free deck is supported on steel plate girders at a spacing of 2.7 m. The transverse confinement in the deck slab is provided by means of 200 x 200 straps welded to the top flanges of the girders at a spacing of 1.2 m. With the barrier walls resting almost directly above the outer girders, the deck slab has very small cantilever overhangs. As can be seen in Fig. 2, the concrete barrier walls contain inclined steel rods encased in PVC tubes; these rods, bolted at the top of the wall and to the steel cross-frames below the deck, are designed to transfer the vehicle impact loads on the barrier wall directly to the assembly of cross-frames and the girders. Because of the virtual absence of the cantilever overhangs and a direct connection of the barrier wall to the cross-frames, the steel-free deck slab of the Salmon River bridge is altogether free from the transverse negative moments, which are induced by loads on the cantilevers; these negative moments are not accoodated by the arching action in the slab. The Salmon River bridge, located on the Trans Canada highway, is subjected daily to very heavy and numerous vehicles. Despite having been subjected to such severe loads and more than two cycles of the harsh Canadian weather, it shows no signs of distress. In winters, this bridge is subjected to virtually daily freeze-thaw cycles. The unit prices quoted by the contractor for the conventional and steel-free deck slabs were Cdn. $ 143 and 152 per m 2 of the deck slab area, respectively. The main reason for the steel-free deck slab to cost about 6 % more than its conventional counterpart was the fact the contractor, having no experience in fibre reinforced concrete, was apprehensive of the problems associated with this new concrete. The contractor's apprehension was proven unjustified, and as shown later, the steel-free deck slab can be more economical than reinforced concrete slabs on even a first cost basis. Their enhanced durability makes them even more economical on a life-cycle cost basis. It is emphasised that this and subsequent cost comparisons of the steel-free deck slabs are with respect to reinforced concrete deck slabs designed by the empirical method of the OHBDC which takes account 65

4 of the arching action. Deck slabs designed for bending have already been shown to be more expensive than the OHBDC slabs. Fig. 2: Half cross-section of the Salmon River bridge 3.2 Chatham bridge The deteriorated deck slab of the Kent County Road No. 10 bridge in Chatham, Ontario, Canada, was replaced in the Fall of This bridge has two lanes and four spans of 13, 20, 20 and 13rn. It crosses over the very busy Expressway No. 401, and has five steel girders at a spacing of 2.1m. The middle two spans now have a 225 thick deck slab with steel reinforcement designed by the OHBDC empirical method. The two outer spans have a 175 thick steel-free deck slab. This latter slab contains, in the cantilever overhangs and the outer panels, a grid of carbon fibre reinforced plastic (CFRP) placed near its top surface. As can be seen in Fig. 3, this reinforcement is provided only in the cantilever and the first interior panel of the deck slab for the negative moments, which are induced due to loads on the cantilever overhang. Transverse confinement to the deck slab was provided by means of 50x25 galvanised steel straps welded to the top flanges of the girders at a spacing of 1.0m. 66

5 GFRP Grid x 25 Galvanized Steel 1.0 m Fig. 3: Half cross-section of the Chatham bridge deck Because of the use of the expensive CFRP grid for transverse negative moments, the per unit cost of the steel-free deck slab of the Chatham bridge was significantly higher than that of conventional deck slabs. It was found that such an increase in the cost of the deck slab, even if justifiable on the life-cycle cost basis, is unlikely to be acceptable to most bridge owners except for a demonstration bridge or two. 3.3 Crowchild Trail bridge The shortcoming of significantly higher costs for the steel-free deck slab of the Chatham bridge was removed in the deck slab of the Crowchild Trail bridge in the City of Calgary, Alberta, Canada. As shown in Fig. 4, this 185 thick deck slab rests on steel plate girders at a spacing of 2.0 m. Similar to the deck slab of the Chatham bridge, this deck slab also has cantilever overhangs and incorporates barrier walls reinforced with GFRP rods, and connected to the deck slab by means of double-headed tension bars of steel. The reinforcement for transverse cantilever moments is provided by GFRP bars, which are significantly cheaper than the CFRP grid. 67

6 19 dia. DOUBLE - HEADED STAINLESS STEEL Nos. 22 dia. STUDS ON STRAPS GFRP GRID 185 GFRP REINFORCEMENT x 200 STEEL 1.2 m Fig. 4: Half cross-section of the Crowchild Trail bridge deck The transverse confinement to the deck slab of the Crowchild Trail bridge is provided by 50 x 25 straps, which contain welded shear studs directly over the girders, and are spaced at 1.2 m. The girders of the Crowchild Trail bridge are continuous over three spans of 30, 33 and 30 m, because the deck slab is subjected to global longitudinal negative moments above the two intermediate supports. The CHBDC requires that for continuous-span bridges, the steel-free deck slabs contain longitudinal negative moment reinforcement in at least those segments in which the flexural tensile stress due to longitudinal bending at the Serviceability Limit State are larger than 0.6 times the nominal cracking strength of concrete. To conform to this requirement, the deck slab of the Crowchild Trail bridge was provided with longitudinal GFRP bars over the two piers. It is interesting to note that for the bridge under consideration, alternative bids were sought for the conventional as well as the steel-free deck slabs. The steel-free deck slab was selected because of the lower bid price, thus confirming that the more durable slab can also prove to be cheaper. The Crowchild Trail bridge was opened to traffic in the Fall of Waterloo Creek Bridge The fourth application of the steel-free deck slab was on the Waterloo Creek Bridge in British Columbia, Canada; this is an integral abutment bridge with precast concrete girders having a single span of 25m. As can be seen in Fig. 5, the 190 thick steel-free deck slab rests on girders at a spacing of 2.8m. Transverse confinement to the steel-free deck was provided by means of 50 x 25 studded straps at a spacing of 1.25m. The Waterloo Creek bridge was opened to traffic early in

7 LC 4-22 dia. STUDS WELDED TO STRAPS OVER GIRDERS 55 X 26 m GALVANIZED STEEL 1.25 m GFRP 25 dia. DOUBLE-HEADED STEEL L 100 X 100 X 8 GALVANIZED (TYP) L 75 X 75 X 8 GALVANIZED (TYP) Fig. 5: Cross-section of the Waterloo Creek bridge 3.5 Lindquist Creek bridge Forestry bridges in Canada are usually single-lane, single span structures with two steel plate girders and a deck comprising precast reinforced concrete panels, which are made composite with the girders by means of clusters of studs. The deck panels are provided with circular holes to accoodate the clusters of studs. The holes containing the studs and the gaps between the transverse edges of adjacent panels are filled with a quick-setting grout. The bridge can be opened to traffic within 24 hours after the erection of the steelwork. The concept of arching in deck slabs was further utilized in an alternative to the reinforced concrete precast panels for forestry bridges. The alternative, entirely devoid of tensile reinforcement, is illustrated in Fig. 6. Panels, with a 150. thickness at the crown and spanning over girders at a spacing of 3.5 m, were used for the Lindquist bridge on a forestry gravel road in British Columbia. The transverse confinement to the panels is provided by 25 x 50 studded steel straps at a spacing 1.0 m. The straps at their ends are embedded in the precast panels. It is interesting to note that the reason for the development of the precast panels for forestry bridges, instead of durability, was economics. These panels are about 30 % cheaper than the conventional reinforced concrete panels. 69

8 x 250 WOOD BLOCKS x 25 STEEL 1.0 m SECTION Fig. 6: Cross-section of the deck of the Lindquist bridge 4. Analytical method After extensive research, Wegner and Mufti (1994), found that the behaviour of suitably-confined deck slabs can not be predicted with confidence with non-linear finite element methods; they concluded that good comparison with experimental results can be made only after 'tuning' the finite element parameters, which requires a prior knowledge of the experimental results. Following the research noted above, an analytical method was developed which could reliably predict the behaviour of restrained deck slabs with or without reinforcement (Mufti and Newhook, 1998). This method, which is based on an earlier work by Kinnunen and Nylander (1960), explicitly takes account of the various parameters that influence the punching strength of the slab; these factors being (a) girder spacing, (b) effective deck slab thickness, (c) concrete strength, (d) size of load, and (e) the stiffness of the transverse confining system. The method of Mufti and Newhook is included in a program called PUNCH. Failure loads for various steel-free deck slabs tested by the authors are compared in Table 1 with the failure loads given by PUNCH. It ran be seen that the program can predict the failure load fairly accurately. For brevity, only limited information is provided in Table 1. For example, the sizes and spacing of the straps, which varied from model to model, are not noted; for such information, reference should be made to the work by Mufti and Newhook (1998). It is noted that for test Nos. 12 and 13 in Table 1, the effective slab thickness is equivalent to the distance between the crown of the slab and the top of the girder flanges. 5. Patent The practice of patenting 'innovative concepts' in the field of structural engineering is not new. For example, the famous Swiss bridge engineer Robert Maillart had several patents to his credit, including the 1907 patent for the three-hinged reinforced concrete arch, and the 1908 patent 70

9 for the flat slab. In the same tradition, the concepts of cast-in-place and precast steel-free deck slabs are protected through patents. The patent for cast-in-place steel-free deck slabs is already granted in USA, UK and France, and is pending in Canada, Germany and Japan- A global patent for precast arch panels without tensile reinforcement is pending. Table 1: Failure loads for various steel-free deck slabs No. Compressive Strength, MPa Girder Spacing, m Effective slab Thickness, Experimental Failure load, kn Analytical failure Load, kn References Bakht, B.; and Agarwal, A-C., (1995), Deck Slabs of Skew Bridges, Canadian Journal of Civil Engineering, 22(3), pp Kinnunen, S; and Nylander, H., (1960), Punching of Concrete Slabs Without Shear Reinforcement, Transactions, Royal Institute of Technology, Stockholm, Sweden, No Mufti A.A., Jaeger, L.G.; Bakht, B.; and Wegner, L.D., (1993), Experimental Investigation of FRC Slabs Without Internal Steel Reinforcement, Canadian Journal of Civil Engineering, 20(3), pp Mufti, A.A.; and Newhook, J.P., (1998), A Rational Method of Predicting the Behaviour of Laterally Restrained Deck Slabs Without Reinforcement, ACI Structures Journal, To be published. Razaqpur, A.G.; and Ghali A., (1984), Forces at Flange-web Connections in T-beams, Canadian Journal of Civil Engineering, 11(4), pp Wegner, L.D.; and Mufti, A.A., (1994), Finite Element Investigation of Fibre-Reinforced Concrete Deck Slabs, Canadian Journal of Civil Engineering, 21(2), pp